1. Field of the Invention
This invention relates to quality assurance methods used for quality assurance for laser shock peening and, more particularly, for acoustic monitoring and statistical analysis method for quality assurance of a production laser shock peening process.
2. Discussion of the Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP""D to create the beneficial compressive residual stresses. Other techniques to confine and redirect the shock waves that do not use water have also been developed.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. Nos. 5,756,965 entitled xe2x80x9cON THE FLY LASER SHOCK PEENINGxe2x80x9d; U.S. Pat. No. 5,591,009, entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,569,018, entitled xe2x80x9cTechnique to prevent or divert cracksxe2x80x9d; U.S. Pat. No. 5,531,570, entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447, entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329, entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328, entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
LSP is a deep treatment of the material and it is desirable to have a quality assurance test that is indicative of a volumetric LSP effect. It is also desirable to have a QA method that is compatible with a dual sided or simultaneous dual sided LSP process wherein substantially equal compressive residual stresses are imparted to both sides of a workpiece, i.e. along the leading edge of a gas turbine engine fan blade.
One laser shock peening quality assurance technique previously used is high cycle fatigue (HCF) testing of blades having leading edges which are LSP""d and notched in the LSP""d area before testing. This method is destructive of the test piece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP""d components. An improved quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, quick, and easy to set up is highly desirable. It is also desirable to have a real time NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. It is desirable to have a real time NDE method so that process deviations can be discovered during a production run. Therefore, any real time techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
Interferometric profilometry method and apparatus to obtain volumetric data of a single laser shock peened test dimple created with a single firing of a laser used in the laser shock peening process is disclosed in U.S. Pat. No. 5,948,293 xe2x80x9cLaser shock peening quality assurance by volumetric analysis of laser shock peened dimplexe2x80x9d. Other QA methods are disclosed in U.S. Pat. No. 5,987,991 xe2x80x9cDetermination of Rayleigh wave critical anglexe2x80x9d; U.S. Pat. No. 5,974,889 xe2x80x9cUltrasonic multi-transducer rotatable scanning apparatus and method of use thereofxe2x80x9d; and U.S. Pat. No. 5,951,790 xe2x80x9cMethod of monitoring and controlling laser shock peening using an in plane deflection test couponxe2x80x9d. U.S. Pat. No. 6,254,703,entitled xe2x80x9cQuality Control Plasma Monitor for Laser Shock Processingxe2x80x9d discloses a method and apparatus for quality control of laser shock processing by measuring emissions and characteristics of a workpiece when subjected to a pulse of coherent energy from a laser. These empirically measured emissions and characteristics of the workpiece are correlated to theoretical shock pressure, residual stress profile, or fatigue life of the workpiece. Apparatus disclose includes a radiometer or acoustic detection device for measuring these characteristics.
A method for quality control testing or monitoring of the laser shock peening process of production workpieces includes the following steps. Step (a) includes laser shock peening a surface of the production workpiece by firing a plurality of laser beam pulses from a laser shock peening apparatus on the surface of the production workpiece and forming a plurality of corresponding plasmas. Each one of the plasmas for each one of the pulses has a duration in which the plasma causes a region to form beneath the surface. The region has deep compressive residual stresses imparted by the laser shock peening process. Step (b) includes measuring acoustic signal for each of the laser beam pulses during a period of time during the duration of each corresponding one of the plasmas. Step (c) includes calculating an acoustic energy parameter value for each of the acoustic signals for each of the corresponding laser pulses or plasmas. Step (d) includes calculating a statistical function value of the workpiece based on the acoustic energy parameter values. The statistical function value may be an average of the acoustic energy parameter values for the plurality of the laser beam pulses. In step (e) the statistical function value is compared to a pass or fail criteria for quality assurance of the laser shock peening process for accepting or rejecting the workpiece. Besides using the averages of the acoustic energy parameter values to determine the statistical function values other types of statistical functions and analysis may be used, i.e. analysis and functions using regression or standard deviations.
The pass or fail criteria may be based on a pre-determined correlation of test piece statistical function data. More particular embodiments use high cycle fatigue failure based on high cycle fatigue tests of test pieces. The test pieces may have a failure precipitating flaw within a laser shock peened area of the test piece that was laser shock peened in the same or similar laser shock peening apparatus.
Two exemplary types of acoustic signal monitoring devices are disclosed. The first type is an acoustic transducer mounted to the workpiece, which detects acoustic signals though the workpiece. The second type is a microphone located away from the workpiece, which detects airborne acoustic signals. The acoustic signals may be used to calculate various types of acoustic energy parameters of the laser pulse or plasma. One exemplary type of acoustic energy parameter is a maximum amplitude of each corresponding one of the signals during the duration of each corresponding one of the plasmas. A second exemplary type of acoustic energy parameter is a signal from one of the plasmas integrated over time of a sample period of the duration of the plasma and also referred to as the area under the curve of the acoustic signal. The exemplary embodiments describe four separate and distinct acoustic energy parameters that can be calculated during laser shock peening of production workpieces and four corresponding statistical function values that can be correlated to pass or fail criteria based on the same parameters of test pieces.
The surface is typically laser shock peened with more than one sequence of coatings of the surface and then firings of the laser beams on the surface such that adjacent laser shock peened circular spots are hit in different sequences or passes of the laser beams forming layers of overlapping laser shock peening spots. The pattern of sequences entirely covers the laser shock peened surface. The plurality of laser beam pulses or plasmas used in the present invention may be from all or a portion of all of the pulses or plasmas in each layer for the purposes of correlation. Not all of the laser beam pulses or plasmas need be included in the plurality of the laser beam pulses used for the quality assurance method of the present invention. Acoustic data from a portion of the plasmas may be used for the plurality of the laser beam pulses used in method.
The present invention provides efficient, reliable, and repeatable quality assurance testing during laser shock peening production runs. The invention provides a quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, quick, and easy to set up. The method of the present invention provides a real time NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. The real time NDE method of the present invention allows deviations to be discovered during a production run resulting in lower scrap rates and less wasted production time.